By chance are you stopping in St. Maarten? If so, grab a cab and go out to "Orient" beach. There are really 2 beaches right next to each other, one is for the general public and you can go topless and the other is part of a nudist colony.
You can actually go to any one of the 2 beaches and its really a nice place.
There are tons of water sports you can do.
BTW, what cruise ship are you going on and when? Enjoy!